The present invention relates in general to generation of images using x-ray computed tomography (CT) apparatus, and more specifically to correction of x-ray measurements contaminated by crosstalk errors between adjacent detectors in the CT apparatus.
Modern CT equipment reconstructs cross-sectional images of the x-ray attenuation coefficient in a subject using a rotating source of fan-beam x-rays and an array of x-ray detectors for measuring the fan beam after attenuation by the subject at a plurality of rotational positions (i.e., views). In the third-generation CT scanner, a rotating detector array comprises many detector elements (or channels) in side-by-side alignment in the plane of rotation on the opposite side of the subject from the fan-beam source. The x-ray measurements obtained by each of the detector elements in each of the views are combined to form an image using a well-known method, such as filtered back projection.
Since the elements in a detector array must be closely spaced, they are likely to be subject to crosstalk, where x-rays impinging on one channel create output signals in that channel and in adjacent channels. In commonly used xenon-gas detectors, crosstalk is caused by x-ray scatter between detector cells and by leakage of charge between cells. In solid-state detectors, crosstalk is caused by x-ray scatter, leakage of visible light produced in one scintillator into the photosensitive diode associated with a different scintillator, and leakage of electrical signals between adjacent diodes.
Crosstalk creates ring and streak artifacts in reconstructed images from third generation CT scanners because of the distortions in the individual x-ray measurements of each channel. As the name suggests, the ring artifacts appear as bright or dark circles or portions of circles centered on the rotation axis (i.e., isocenter) of the source and detector array. The rings tend to appear near the center of the field of view and in areas of abrupt change in attenuation coefficient. Streak artifacts appear as bright or dark lines tangential to the edges of dense (i.e., high x-ray attenuating) objects.
Previous efforts to reduce ring and streak artifacts include the use of matched detectors having a high degree of uniformity between elements. If crosstalk characteristics for all adjacent channels are substantially equal, then accumulated crosstalk errors in each channel will approximately cancel since x-ray intensity variation across the detectors during a view is approximately piecewise linear. However, matched detector arrays are difficult to implement and are expensive.
Another approach has been to inspect a reconstructed image for ring artifacts and manipulate the image in an attempt to remove the rings using numerical methods. However, this approach cannot remove streak artifacts. Furthermore, ring artifacts beyond a certain severity cannot be removed.
Accordingly, it is a principal object of the present invention to reduce crosstalk artifacts in CT images without the use of special matched detectors.
It is another object to provide a method and apparatus to reduce all artifacts in CT images caused by crosstalk, including ring artifacts and streak artifacts.
It is a furtherobject to characterize the crosstalk of a detector array in a simple and convenient manner, without special preparation or hardware.
It is another object to remove crosstalk errors from CT data prior to image reconstruction.